The Cameroonian keeper, signed from Italian side Inter Milan last month, was beaten from long range as he made his first appearance at Old Trafford.
Right-back Diogo Dalot gave the ball away just inside the United half and Lens striker Florian Sotoca spotted that Onana was off his line before lobbing the back-pedalling keeper to open the scoring after 22 minutes of the pre-season friendly.
Marcus Rashford levelled the scores in the 50th minute after he ran on to a superb through ball by Brazilian winger Antony before beating a defender and scoring via a slight deflection.
Antony, who had a superb game, then turned from provider to goalscorer three minutes later.
Rashford played a lovely ball around the corner to release Alejandro Garnacho, who found Antony on the right flank for the winger to guide his shot past Brice Samba in the Lens goal.
United then added a third goal in the 58th minute when a Rashford header went in off Brazilian midfielder Casemiro, who was credited with the goal.
United manager Erik ten Hag fielded a strong starting line-up and brought on Jadon Sancho and Omari Forson late in the game.
Mason Mount, also making his first appearance at Old Trafford, missed a sitter in the first half to make it a bad day for United's new boys.
Garnacho picked up a cross-field pass from Bruno Fernandes, who laid the ball off to Mount but the midfielder managed to miss in front of an empty net.
Perhaps the most significant action of the day took place before the kick-off as United's new Danish striker Rasmus Hojlund was unveiled on the pitch after completing his £72 million signing.
The Red Devils face Athletic Bilbao on Sunday in their final pre-season friendly in Dublin before their Premier League season kicks off at home to Wolves on Monday, August 14.