Maezawa began attending Waseda Jitsugyo High School in 1991, where he started up a band with his classmates called Switch Style. The band released their first EP in 1993. After graduating from high school, he decided not to go to college; instead he moved to the US with a girlfriend, where he started collecting CDs and records. When he returned to Japan in 1995, his album collection became the basis for his first company, which sold imported albums and CDs through the mail.

In 1998, Maezawa used the basis of the mail-order album Business to launch the company Start Today. The same year, his band signed with the label BMG Japan. By 2000, Start Today had moved to an online platform, had begun selling clothing, and had become a public company. In 2001, Maezawa declared a hiatus on his music career. Start Today opened the Retail clothing website Zozotown in 2004, and six years later, Start Today became a publicly traded company, listed on the "Mothers" Index of the Tokyo Stock Exchange. By 2012, Start Today was listed in the First Section of the Tokyo Stock Exchange.

Maezawa is the founder of the Tokyo-based Contemporary Art Foundation, which he started in 2012 with a goal of "supporting young artists as a pillar of the next generation of contemporary art." The Contemporary Art Foundation currently hosts collection shows twice a year. In May 2016 Maezawa attracted significant media attention with a record purchase price at auction of $57.3 million for an untitled work by Jean-Michel Basquiat, and broke a record again in May 2017 with a $110.5 million auction for a piece by the same Artist. At the same 2016 auction, Maezawa bought pieces by Bruce Nauman, Alexander Calder, Richard Prince, and Jeff Koons, spending a total of $98 million over two days. Maezawa plans to open a contemporary art museum in Chiba, which will house his collection.