Defending champions Portugal defeated Luxembourg's national team 2-0 at Josy Barthel stadium this Sunday.
Portugal became the 17th nation to qualify for next year's multi-host tournament, which kicks off in Rome on June 12, joining the likes of world champions France, Spain, Italy and England.
Bruno Fernandes struck the opening goal on 39 minutes before Ronaldo tapped in a scrappy second late on to clinch second place in Group B behind winners Ukraine, who drew 2-2 in Serbia.
Cristiano Ronaldo scored his 99th international goal as reigning champions Portugal secured their place at the Euro 2020 finals on Sunday with a 2-0 victory away to Luxembourg.
Fernando Santos' side knew they would book their place at Euro 2020 with a win or by matching Serbia's result from their game in Belgrade.
But Portugal were made to work hard on a heavy pitch by a Luxembourg team ranked 96th in the world that notably held France to a 0-0 draw in 2018 World Cup qualifying.
Sporting Lisbon midfielder Fernandes broke the deadlock before half-time, controlling Bernardo Silva's pinpoint pass expertly before lashing in from 20 yards.
Ronaldo, who smacked in a hat-trick in Thursday's 6-0 thrashing of Lithuania, moved to the brink of his international century four minutes from time.
Silva's far-post cross was forced towards goal by Diogo Jota, with Anthony Moris getting a hand to the ball before Ronaldo turned in from the goalline.
Iranian striker Ali Daei holds the world record with 109 international goals scored between 1993 and 2006.