1st marriage to W. M. McCartney 4 Jul 1888 Eureka, Greenwood, Kansas. 2 children born to their union.

2nd marriage to Arthur P. Johnston 30 Sep 1900 Missoula, Missoula, Montana. 1 child born to their union.

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The following information is provided by Find A Grave contributor RunninonMT.

Since Mary L. Cockrell and her brother John Joel Cockrell were both born in 1866 it's presumed that they are twins.

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"Montana, Mineral County Obituaries, 1870-2010," database with images, Family Search - Mrs. Mary Lydia Cockrell Johnston, 1943:

"Mrs. Mary L. Johnston, Western Montana pioneer, died March 4, 1943, in Helena, Montana, and funeral services were held in Helena with Rev. O. C. Hansen officiating."

"Mary Lydia Cockrell, born at Neosho, Missouri, January 31, 1866, the daughter of Keziah Church Cockrell, a direct descendant of the Church's of Virginia who came to America on the Mayflower, and Thomas J. Cockrell, a native of Washington, Indiana who was a brother of the late Francis Cockrell, long a prominent United States Senator from Missouri. Thomas J. Cockrell, her father, was a Union soldier during the Civil War, and to his death in 1916 was a member of the Grand Army of the Republic."

"On July 4, 1888, at Eureka, Kansas, Mary Lydia Cockrell married William McCartney, and thereafter were born two children: Leta Margaret McCartney, a daughter, and Berna William McCartney, a son. Mrs. Mary L. (McCartney) Johnston arrived in Superior, Montana, with her first two children on March 4, 1892; and married Arthur P. Johnston at Missoula, Montana, on September 30, 1900. To them was born a daughter, Grace Adelaide, now Mrs. Edward Horsky of Helena, Montana. Mr. Johnston died January 27, 1930."

"Mrs. Johnston is survived by all three of her children, namely; Mrs. B. J. Slade of Memphis, Tennessee, a son, Berna W. McCartney of Spokane, Washington, and Mrs. Edward Horsky of Helena, Montana. She is also survived by two sisters, Mrs. Lizzie Miles of Superior, Montana, Mrs. Lina Ames of Manhattan, Kansas: and two brothers, Lum Cockrell of Whitefish, Montana, and Grant Cockrell of Orlando, California; also many nieces and nephews in Kansas, Oklahoma, California, and Montana."

"Mrs. Johnston resided at the ranch home, one mile east of Superior, but was spending the winter with her daughter, Mrs. Horsky in Helena, at the time of her passing. The Johnston ranch was the first homestead in the vicinity of Superior, having been purchased by Mr. Johnston in the early '80s soon after his arrival in Montana."

Several notes of clarification:
•Thomas J. Cockrell was born near Livonia in Washington County, Indiana, and served in the Missouri State Militia on the Union side during the Civil War. He was not a brother to Sen. Francis Marion Cockrell (FAG memorial #9913), but a first cousin - their grandfather was the Rev. Simon B. Cockrell, abt. 1745-1835.
•Mrs. Lina Ames is Amanda Pearlina Cockrell Ames; Lum Cockrell is Columbus Jefferson Cockrell; and Grant Cockrell is Ulysses Grant Cockrell.
•As of April 2019, no proof has been found by this writer to either confirm or deny the claim that Keziah Church's lineage came to America on the Mayflower.