David Martindale's side made the trip to Fir Park knowing that a loss, coupled with Ross County avoiding defeat at home to Hibernian, would open up an unassailable gap to the relegation playoff spot.
Livingston were comfortably beaten in the end, losing for a 22nd time this season, while Ross County picked up a crucial 2-1 win to climb up to 10th and leave St Johnstone now in the relegation playoff place.
Motherwell opened the scoring midway through the first half courtesy of a penalty from Thelonius Bair, before Sam Nicholson doubled their advantage on the stroke of half time.
Bair added a second in the 68th minute to effectively secure the win that ensures Motherwell's top-flight status for another season, and Dan Casey poured more misery on the visitors with a fourth six minutes later.
Jon Nouble struck the meerest of consolations for Livingston moments later, but by that stage their best hopes of surviving at least another matchday lay with Hibernian at Dingwall.
Hibs did take the lead in that match through Myziane Maolida, only for Simon Murray to restore parity just six minutes later for Ross County.
A draw for Don Cowie's side would have sent Livi down anyway, but they gave their own chances of avoiding a relegation playoff a huge boost late on as Jordan White struck an 88th-minute winner.
St Johnstone and Ross County are set to meet in a crucial showdown on the penultimate weekend of the season.