The 800th goal came in the 52nd minute of the match at Old Trafford when Marcus Rashford picked out Ronaldo on the right side of the box, who shot first time into the far bottom corner from about 10 yards out.
Ronaldo then scored No. 801 when he converted from the penalty spot in the 70th minute.
The goals came in the first Premier League match at Old Trafford since Ole Gunnar Solskjaer left his role as United manager, with incoming interim coach Ralf Rangnick watching from the stands.
Ronaldo, who has previously finished as top goal scorer in England's Premier League, Spain's LaLiga and Italy's Serie A -- while also winning the respective leagues and domestic cups on multiple occasions -- is also the man with the most Champions League medals in the modern era with five victories.
Ronaldo made his professional debut at SportingCP in 2002 and scored his first two professional goals in a 3-0 victory over Moreirense on Oct. 7 of that year. He joined United at the end of the season after scoring five goals in all.
He spent six seasons during his first spell in Manchester, scoring 118 goals in 292 appearances before signing for Real Madrid in a then-world record fee of £80 million.
During Ronaldo's nine seasons in the Spanish capital, he scored an astonishing 450 goals in 438 appearances across all competitions to become the club's all-time leading goal scorer.
He moved to Juventus in 2018 in a deal worth £100m and netted 101 goals in 134 matches for the Bianconeri. Ronaldo returned to United in August and has so far scored 12 goals to take him to 130 in total for the club.
Ronaldo's 800th goal came just over two years after he reached 700, and he has scored 338 goals in 372 matches since turning 30.