Across 8 innings, Pollard aggregated 51 runs with only one double-digit score (26 in 2015 T20I).
Fate intervened briefly when a floodlight failure halted play, but the respite was temporary as, despite resistance from Keemo Paul and Fabian Allen, the West Indies were bowled out for 184 with Wanindu Hasaranga and Lakshan Sandakan claiming 3-30 and three for 57 respectively.
Sri Lanka took an unbeatable 2-0 lead in the three-match series after edging the first game by one wicket.
But Fernando and Mendis paired in a rollicking, record 239-run, third wicket stand to all but dash West Indies' hopes of staying in the series, following their opening defeat last Saturday.
Ambris, who was dropped by Sandakan on seven off paceman Nuwan Pradeep, was the first to depart after being run out for 17.
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As pressure was mounting due to the lack of boundaries, Mitchell Marsh fell victim to Lungi Ngidi's delivery in the 18th over. The extra run, from the no-ball call, stood though as Australia went in search of their second win in the series.
The highest third-wicket partnership is stitched by West Indies' Dwayne Bravo and Denesh Ramdin as they scored 258-run against Bangladesh in 2014.
Man-of-the-Match Fernando, with 127 off 123 balls with 10 fours, and Mendis with a run-a-ball 119 and 12 fours, had earlier pummeled West Indies to propel the home side to 345 for eight off their 50 overs.
Darren Bravo (16) misread a googly from Hasaranga for a catch to Dhananjaya de Silva at slip and the Sri Lankan spinners then picked up regular wickets as the West Indies was bowled out in the first ball of the 40th over. It was Sri Lanka's highest third-wicket ODI partnership against any team and Sri Lanka's third-highest ODI partnership for any wicket against any team.
Among the West Indies bowlers, Sheldon Cottrell (4/67) took four wickets, and Alzarri Joseph (3/57) scalped three wickets.
The final match of the series will be played at Pallekele International Cricket Stadium on March 1. The island nation also recorded their biggest win in terms of runs against the West Indies. The previous best was 333 by England against Australia in Sydney in 2011.