At lunch, England, who had been 56 without loss, were 71 for three after paceman Lakmal (two for 22 in seven overs) had accounted for both Nick Compton (one) and Joe Root (three).
England captain Alastair Cook, who won the toss in ideal sunny batting conditions and on a placid pitch, was 48 not out, with James Vince unbeaten on three.
England had already won this three-match series at 2-0 up.
In the build-up Cook insisted they were determined to correct their habit of losing 'dead' Tests in series they'd already won following heavy defeats at the end of victorious campaigns at home to Australia in 2015 and away to South Africa earlier this year.
Also Read
Sri Lanka, who retained Shaminda Eranga in their side despite the paceman being reported for a suspect action after concerns emerged in the second Test, made little impression as Cook and Alex Hales compiled a fifty stand in 74 balls.
Left-hander Cook, presented with a commemorative bat before play to mark his achievement in becoming the first England batsman to score 10,000 Test runs -- a landmark he reached at the Riverside -- leg-glanced Eranga for four.
But Rangana Herath succeeded where the pacemen had failed when, with just his fourth ball, the left-arm spinner had Hales (18), slogging across the line of a ball that turned, caught by Sri Lanka captain Angelo Mathews at slip.
By his own admission, Compton was playing for his Test place.
But on his Middlesex home ground he fell when he was caught behind after failing to get to the pitch of a gentle Lakmal away-swinger.
Lakmal then made it two wickets for two runs in five balls when he had Root (three) lbw, hitting across the line, although Sri Lanka needed to review Australian umpire Rod Tucker's original not out decision.