Delray Rawlins hit his maiden first-class century and shared an exhilarating sixth-wicket partnership of 128 with David Wiese at Emirates Old Trafford but he could not prevent Sussex losing their Division Two match against Lancashire by an innings and 51 runs.
The Red Rose sealed victory seven balls into the evening session on the third day at Emirates Old Trafford.
Unbeaten Lancashire’s 23 points for their victory left them 30 points clear in Division Two of the County Championship.
Second-placed Glamorgan’s match at Middlesex is still in progress but Sussex’s third successive defeat puts a severe dent in their hopes of winning promotion.
Matt Parkinson took 4-142 in Sussex’s second innings to finish with career-best match figures of 10-165.
Needing 367 to make the home side bat again, the visitors were bowled out for 316, with Onions taking the ninth wicket when he had Abi Sakande caught behind by Dane Vilas.
Parkinson ended the match when he trapped Mir Hamza leg before for nought.
But the highlight of Monday’s cricket was undoubtedly the batting of Rawlins, who hit 12 fours and three sixes, reaching his century off 99 balls with his third maximum, a blow which was still rising just before crossing the rope in front of the pavilion.
Rawlins was dismissed next ball, when Parkinson clung on to a sharp return catch, and the leg-spinner took his third wicket when he had Wiese leg before wicket four overs later for 77.
Lancashire’s only other wicket in an afternoon session, which saw 197 runs scored, was taken by Blackpool’s Richard Gleeson, who had Chris Jordan caught behind by ilas for 11.
But all this entertainment and the aggression of Rawlins and Wiese was in contrast to the opening hour of play, in which Sussex’s hesitant and error-strewn batting resulted in the loss of four wickets.
Varun Chopra was the first to go for 15, when he nibbled at a ball he probably need not have played at from Gleeson and was caught behind by Vilas.
Less than three overs later, Luke Wells was leg before wicket to Onions for a third-ball duck and Stiaan van Zyl followed for six soon after.
His flick down the leg-side off Gleeson only edged another catch for Vilas.
The important dismissal of visiting skipper Ben Brown, caught at midwicket by Haseeb Hameed for four when chipping a ball from Onions into the legside, meant that Sussex had lost four wickets inside the first hour.
Phil Salt was then bowled by Parkinson for 37 on the stroke of lunch, when he played the ball down into a foothole and it spun back into the off-stump.
At that point Sussex seemed likely to lose heavily but Rawlins’ batting brought a smile back to the faces on the away dressing room balcony.
Onions finished the match with 3-18 and Gleeson took three 3-62.
Lancashire now revert to T20 mode for the Vitality Blast, starting on Friday with a visit to Leicestershire.