George Mason was an 11 seed in the 2006 March Madness and a 12 seed in the 2008 March Madness.
The lowest seeded team to make it all the way to the Final Four in the NCAA basketball tournament was an 11 seed and it happened four times:11 seeded LSU in 198611 seeded George Mason in 200611 seeded VCU in 201111 seeded Wichita State in 2013
3 #11 seeds have made the Final Four. LSU in 1986 George Mason in 2006 VCU in 2011
The answer to your question is yes. George Mason (2006) and LSU (1986) both beat #1 seeds as #11 seeds. In 2011, Virginia Commonwealth (VCU) beat #1 Kansas to become the third #11-seed (LSU and George Mason being the first two) to advance to the Final Four.
The lowest seed to ever win a NCAA Basketball title was Villanova, ranked seed #8. They won the tournament in 1985. Lower seeds have made it to the Final Four (George Mason and Louisiana State, ranked 11th, made it in 2006 and 1986 respectively) but never won. UCLA in 1980 was ranked 48th and made it to the championship game.
They were an 11 seed.
George Mason University
11 seed George Mason
George Mason 2006 LSU 1986