I cannot find a death record for Mary, but her son Hiram, with whom she was living in 1920, was back in Lincoln Co., West Virginia in 1930, so she probably died there sometime between 1920 and 1930.

In 1900 she was listed as age 46, b. Nov 1853, widowed, with her daughter Lillie in Lincoln Co., WV.

I cannot find her in the 1910 census. Most likely she was still living with Lillie, whom I also cannot find in 1910.

In 1920 she was listed as age 66, widowed, with her son Hiram in Lewis Co., Ky.

Wyatt and Mary Ann most likely separated shortly after the birth of Lillie. When Wyatt remarried to Elizabeth Cline in 1897, he reported it as his first marriage, and in the 1900 census, Mary reported herself as widowed.

1880 census of McDowell Co., West Virginia, Sandy River district, enumerated 30 June, lists on p. 19 as family #151/157:
Lambert, Wyatt S., 34, farmer, b. Va Va Va; Mary Ann, 26, wife, b. Va Va Va; Mollie, 10, dau; Rebecca, 7, dau; Hiram, 3, son; Cornelius, 2/12, b. March, son. All children born WV Va Va.

The preceding household is that of Mary Ann’s brother, L.D. Trent with wife Nicatie, Wyatt’s sister, and the following household is headed by Wyatt’s widowed mother.

Mollie is probably really only 9, because in the 1870 census, both Wyatt and Mary were recorded as single and living with their respective parents.