She became the first lady of Florida as wife of Governor William Sherman Jennings and is credited with having advanced his political career significantly through relationships gained while working for her father and through her many activities.

[3]: 30  Austin Mann then ran for the state House of Representatives and May assisted in the campaign, hosted teas, and spoke with people at rallies.

[4] May Mann met Hernando County Judge William Sherman Jennings at her father's home near Brooksville, and a courtship began.

Following her husband's term as governor, the couple moved to Jacksonville, then Florida's largest city, where he established a successful law practice.

[8] With the League, Jennings successfully campaigned for universal cattle dipping in Florida starting in 1923 (to combat tick-borne bovine babesiosis),[9]: 228–231  but lobbied unsuccessfully in 1929 and 1931 for a 48-hour work week for women.

[3]: 292–295  She was an organizer in state Democratic politics[3]: 300–301  and served as campaign manager for Ruth Bryan Owen in her successful 1928 run for House of Representatives.

[3]: 303–304  In securing federal Civilian Conservation Corps workers for Royal Palm State Park during the Great Depression, Jennings cooperated with her son, Bryan, who had become president of the Florida Board of Forestry.

[3]: 305–307  She was the long-time president of the Duval County Highway Beautification Association from 1928 until 1958, having written state legislation, enacted in 1931, that reserved a portion of roads' right-of-ways to plantings and conservation.