Having arrived as part of Michael Duff's backroom staff, Sheehan took over as caretaker boss in December 2023, helping Swansea claim 11 points from seven matches before the appointment of Williams in January last year.
He returned to the backroom team before stepping up to the top job for a second time when Williams said his farewells.
At the time, Swansea were just eight points clear of the bottom three in 17th, with relegation concerns mounting after a run of seven defeats in nine league games.
But they have since delivered a remarkable upturn in form to climb to 11th in the table, 14 points clear of the drop zone and only six adrift of the top six.
Sheehan's side go to Millwall on the back of five straight victories, the club's longest league winning run since Roberto Martinez's Swansea registered six successive third-tier triumphs in 2007.