Mabel Jane Pettit was born on March 24th, 1894 to John Edward Pettit and Emma Matilda Wilde at Almy Wyoming. Mabel's father was in the coal industry, a career which kept them moving periodically. Mabel's mother was from Coalville so when the opportunity came to run the Church's coal mine in Grass Creek Canyon near Coalville, they took it. During a lull in the Church mine's productivity due to flooding the family moved to Wyoming for work, but longed to be back home. So when the chance presented itself, they moved back to Grass Creek again to work at the Church's mine. Her father's skill and reputation gained him an appointment as the General mine Inspector for Utah. That position allowed him to live where he chose, and the decided on Coalville. It was in Coalville where Mable graduated from 8th grade in 1909. 
Sister Pettit served in the Southern States Mission from June 13th, 1915 to March 26th, 1917. She started in Atlanta in the Georgia Conference, where she worked with Sisters Hamilton and Huber. In Atlanta their work included creating positive links with other Churches. A report described one such effort “Sisters Pearl Hamilton and Mabel Pettit, previous to Sister Pettit's transfer to the Florida Conference, upon invitation, attended a social of the Lady Missionaries' Society of the Baptist Church. They were treated with every courtesy and invited to return and pay them another visit at any time.”
She transferred to Jacksonville in the Florida Conference on September 7th, 1915 and worked with Sister Peterson and Rindlisbacher. The President of the Florida Conference wrote that “One [Jacksonville] lady said to them, "I thank the Lord that He sent you dear girls here this morning."  They didn’t just stay in Jacksonville. They also extended their efforts to nearby St. Augustine, Florida.
Then she transferred back to Atlanta on May 28th, 1916 with Sister Rindlisbacher. She was quickly transferred to South Carolina on July 25th 1916 and assigned to work in the Greenville Branch with Sister May Ricks. There she also met Elder McNeill and his companion Elder Hammer. Their work in Greenville, South Carolina could be considered typical of the work they were expected to do everywhere. They tracted, selling Books of Mormon as well as other books and tracts. The ran the YLMIA (Young Ladies Mutual Improvement Association), Sunday Schools, and Relief Societies, organizing them when necessary. The sisters traveled more in South Carolina than perhaps in other Conferences. They visited the Indian Nation or Roddy Branch, as well as Seneca, and Liberty.  They were expected to use any talents they possessed. Mission records indicate Mabel played the organ and she did so for a meetinghouse dedication in Sarah, Mississippi.
This photo of Sis Pettit and her South Carolina companion
was published in the LDS Church’s missionary newsletter
Liahona: The Elders Journal Volume 14 page 366
Mabel was transferred to Middle Tennessee, along with Nellie Rindlisbacher as the first two sister missionaries in the Conference on December 8th, 1916. Although reports only indicate that they "canvassed", and held meetings. It was likely they did many of the same things they did in other conferences.
This photo of Sis Pettit and her fellow Middle Tennessee
missionaries was published in the LDS Church’s missionary
newsletter Liahona: The Elders Journal Volume 15 page 126.
Missions don't last forever. For Sister Pettit it ended on March 26th, 1917. Her return was noted in the local paper  which also referred to her father by the title of Bishop (Coalville Ward 1912-1918). By June she had moved to Salt Lake City, and her visits home were mentioned in the papers.  She obtained a position beginning in the Fall of 1917 to teach school in Coalville. 
In February 1918, Henry "Elmer" McNeill began visiting Mabel in Coalville.  He also served in the Southern States Mission and spent his whole tenure in the South Carolina Conference from Oct 12th, 1915 to January 27th, 1918. Elmer was from New Mexico, not Utah, so it is likely they met for the first time on their mission in South Carolina. Mabel married Elmer on June 12th, 1918. The couple settled in American Fork, Utah. They had two children.
From there Mable disappears from the public record. Little was written about her, though she did write about her parents, from which I have gleaned some of this information. Mable died on December 23rd, 1985 in American Fork, Utah.
 Park Record 1909-05-29 Untitled
 Liahona: The Elders Journal, 13:63
 Liahona: The Elders Journal, 13:141 & 189
 Liahona: The Elders Journal, 13:286
 Liahona: The Elders Journal, 13:237 & 606
 Liahona: The Elders Journal, 13:286
 Liahona: The Elders Journal, 13:731
 Liahona: The Elders Journal, 14:236
 Liahona: The Elders Journal, 14:284
 Liahona: The Elders Journal, 14:349 & 399
 Southern States Mission Manuscript Index, Page 301.
 Liahona: The Elders Journal, 14:447
 Southern States Mission Mauscript Index, Page 301.
 Park Record 1917-04-06 Coalville Notes; Salt Lake Tribune, 1917-04-08, News of the Women's Clubs
 Park Record 1917-06-01 Coalville Notes; Park Record 1918-01-11; Park Record 1918-01-25 Coal Ville; Salt Lake Tribune, 1917-06-03, Social Nortes from Utah Towns.
 Salt Lake Tribune, 1917-05-20, Teachers Assigned for Ensuing Year
 Park Record 1918-02-08 Coalville
See also Emma Matilda Wilde Pettit Biography by Mabel Pettit McNeill