According to the Elmwood Burial Index he died of congestive chill.

Poston was an attorney and a member of the bar in Memphis. He was a charter member of the Independent Order of Odd Fellows, Chickasaw Lodge no 8.

He was the son of John Hamill Poston (1786-1848) and Nancy Lowthing Nelson (1792-1867). Both died in Clarksville Tennessee. He is the grandson of the Revolutionary War soldier, Robert I Nelson. William married Mary Letitia Park on April 13, 1843 in Memphis.

John Hamill Poston: His Ancestors and Descendants, A Poston Family Genealogy-
The Poston home was four miles from Memphis and sat on a five acre site on what is now the intersection of Kerr Ave. and Mississippi Blvd. Poston purchased a old two story house and had it remodeled in 1858. The rooms were large and square in the plantation style of the period.

Mary's mother, Jane Rhodes Barron Park, visited the home each spring. She was very strict and wouldn't allow any of her grandchildren to do fun things like fishing until they had each helped with the spring gardening.

The War between the States affected this home greatly. Skirmishes were played out on the lawns and Memphis was taken in 1862 and remained under Union control throughout the rest of the war. The Poston home was often used by the Union soldiers as a meeting place. William Poston heard and saw a lot but did not pass on anything he heard to the Confederates earning him the trust of the Union soldiers. Because of this he was allowed to pass through the lines and taking food and clothing to his three sons serving in the Confederate Army and when his sons David and James were injured, they were allowed to nurse them in the Poston home.

It was on the second floor of the Poston home that the family silver was hidden from the Union soldiers. Many times throughout the occupation Mrs. Poston would send out Sunday dinner to the Union soldiers in the hopes that "someone is feeding my boys the same way."

The family continued to live in the house after the death of William King Poston in 1866. In 1882-83 W.K. Poston Jr and his family called the place home. In 1893 the home was rented out and was eventually sold in 1898.
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The diary of an old lawyer: or, Scenes behind the curtain
By John Hallum
William K. Poston was a very prominent, able, and sound civil lawyer, and occupied a front seat when I came to the Bar, and had an extensive practice. He was upright in life, an exemplar in the performance of duty, pious, religious, a deacon of the Methodist church, a kind husband, indulgent father; the world was better in every sphere where he moved. Above the medium in size and height, fine physique, with hair inclined to the blonde type. He was dignified and reserved, and a little austere. I never heard him tell a joke or saw him in hilarious laughter.
In July 1866, I was in Lebanon, Tenn, on my summer's vacation, when I was startled, astonished, sorrowed; the papers announced his sudden demise the previous day, the result of a congestive chill. Two fine sons took their father's place at the Bar
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The Daily Union and American
July 12 1866
The Late W.K. Poston
The community of Memphis, who knew the deceased so well and admiringly, paid the highest honors to his remains and memory. Every respect was manifested on the occasion of his funeral. We present below the tribute of the Bar of Memphis, of which he was a distinguished ornament, to his ability and virtues:

We are called together upon a most sad and melancholy occasion. Death has again visited our professional circle, and summoned from all earthly cares and honored and esteemed friend and brother. William K. Poston is dead. In the midst of his usefulness as a citizen, in the zenith of his career as a lawyer, at a period of time when his loss is almost irreparable to the people; not only of this immediate community, but to the entire state, he has, by the inscrutable decree of Providence, been suddenly taken away from us.

Mr. Poston was a native of Montgomery county, in this State, where he was born in 1819. He was a graduate of the University of Nashville, and soon after his graduation commenced the study of law, and upon going through a regular course was admitted to the bar. In the year 1839 he located in this city, then a small village, with a view to the practice of his profession.

At the time of his death he was the oldest surviving member of our bar. For a quarter of a century, he had been among us, diligently, faithfully and earnestly pursuing his profession, and, at the period of his death, had attained a professional eminence and distinction second to that of none of us. As a lawyer, he was always courteous, dignified in bearing towards the courts and his brethren of the bar, and enjoyed the highest respect and esteem of both. he was unusually attentive to all professional business, regarding it as his solemn duty to devote all his capacity and skill to the interests committed to his charge.

As a citizen, he was one of the most exemplary that ever resided in any community. A christian gentleman during the whole of his manhood, he so kindly and so humbly demeaned himself in the performance of all Christian duty as to acquire the love and affection of his own immediate Christian, and of all other sects. As a husband and a father, it is hardly proper to speak in this place. To the sorrowing family he has left behind, no language of this earth can ever bring consolation. They will seek and find it from that high source whence alone can come consolation and peace. Our Almighty Father, "who tempereth the wind to the shorn lamb," will watch over and protect them, and bring, through His ever-enduring love, comfort to their bereaved and stricken hearts.

But the noblest record Mr. Poston has left, is to be found in his career in this city during its occupation by military authority. With a firmness almost unequalled, he resisted all military and arbitrary usurpation, not only in his own person, but in behalf of all who were unfortunately subject to its power. He stood as a "Stonewall" between military authority and his friends, and all others who called on him for assistance. Honest, firm, dignified and calm, he commanded and received the esteem and respect of the military rulers of this city, and thereby became the source of aid to many suffering citizens of West Tennessee and North Mississippi, and when the sad news of his death shall come, many an eye will be dimmed and many a heart be saddened at the rememberance of his many kindnesses and great virtues.

The death of such a good and virtuous citizen, such an exemplary Christian, and so highly esteemed and respected brother, is a very great loss to this entire community; therefore;

Resolved, that we regard the death of William K. Poston as a most sad and grievous affliction to the bar of Tennessee, to the Christian community, and to the entire state at large.

Resolved, that we deeply sympathize with the afflicted family in this, the hour of their bereavement, and offer to them our sincerest condolence.

Resolved, that the Chairman of this meeting appoint a member of this Bar to present these resolutions to each of the courts now in session in this city, the Chancery Court, the Law Court, and the Criminal Court, the Circuit Court of Shelby, and also the Circuit Court of the United States, at its next session in this city, with a request that a copy of the same, under the seal of the court, be transmitted to the family of the deceased, by the Clerk of each said courts.
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Elmwood Cemetery Biographical Sketches, 1874

William King Poston was born at Clarksville, Tennessee, whence he came to Memphis, more than thirty years ago. He rose steadily in the ranks of his profession until, at the date of his death, there were few if any attorneys or barristers deemed his superiors. He was an accurate, logical thinker, and spoke with great facility. His manner, when addressing a jury, was easy and graceful, his voice smooth, even musical; his gesticulation natural and attractive; his expression of face eminently pleasing. He was drawn at last into the political arena, and in conjunction with Sam. P. Walker, in 1866, represented Shelby county and the city of Memphis in the lower branch of the State Legislature. It is needless to say that abler, worthier men have rarely if ever occupied seats in the capitol of Tennessee. Mr. Poston's habits were faultless. He was a devout Christian, devoted to books, and especially to professional learning. The bar of Memphis has not lost a worthier member, or society a more agreeable gentleman, or the State a better citizen. he died in 1866, in the forty-seventh year of his age.