His acting roles include Mikey Walsh in The Goonies (1985), Billy Tepper in Toy Soldiers (1991), Daniel Ruettiger in Rudy (1993), Samwise Gamgee in The Lord of the Rings trilogy (2001–2003), Doug Whitmore in 50 First Dates (2004), Bill in Click (2006), Lynn McGill in the fifth season of 24 (2006), Oso in Special Agent Oso (2009–2012), Raphael in Teenage Mutant Ninja Turtles (2012–2017), Bob Newby in the second season of Netflix's Stranger Things (2017),[1] and Ed in No Good Nick (2019).

[15] Astin attended the Crossroads High School for the Arts and undertook master classes at the Stella Adler Conservatory in Los Angeles.

An alumnus of Los Angeles Valley College, Astin served on the school's board of directors of the Patrons Association and the Arts Council.

Patrol, opposite Nia Long, Tim Thomerson, and Brian Keith; Like Father Like Son (1987); White Water Summer with Kevin Bacon (1987), The War of the Roses (1989); the World War II film Memphis Belle (1990); Toy Soldiers (1991); Encino Man (1992); and the college football biopic Rudy (1993), about the life-changing struggles and rewards of the titular character, Daniel Ruettiger.

In 2004, Astin released There and Back Again (ISBN 0-312-33146-0), a memoir (co-written with Joe Layden) of his film career with emphasis on his experiences during production of The Lord of the Rings trilogy.

[20][better source needed] In television, Astin guest-starred as Lynn McGill in ten episodes of the fifth season of the Fox drama 24.

Astin also voiced Raphael in the Teenage Mutant Ninja Turtles animated series on Nickelodeon, which premiered on September 29, 2012, running for five seasons and 124 episodes, and ending on November 12, 2017.

[23] Also as of 2010, Astin and his wife, Christine, were making a film based on Lois Lowry's Newbery Medal-winning novel Number the Stars.

[26] In March 2012, Astin played a cosmetic surgeon named Takin Mastuhmik in a fake trailer entitled Boobathon which appeared on Funny or Die.

[27] In March 2014, Astin played a soldier in Boys of Abu Ghraib, a military thriller inspired by the events that made worldwide news in 2004.

[29] In October 2015, Astin played Hank Erwin in Woodlawn, a story about how a high school football team overcame racism and hate, and found unity and success through following Jesus.

Others in the episode were fellow The Lord of the Rings castmates Sean Bean, Orlando Bloom, Billy Boyd, Ian McKellen, Dominic Monaghan, Viggo Mortensen, Miranda Otto, John Rhys-Davies, Andy Serkis, Liv Tyler, Karl Urban, and Elijah Wood, plus writer Philippa Boyens and director Peter Jackson.

[33] Astin made his Broadway debut as Santa Claus in a revival of Elf the Musical, which ran at the Marquis Theatre from November 2024 to January 2025.

[37] Alexandra had a small cameo role in The Lord of the Rings: The Return of the King as Elanor Gamgee, Astin's character's daughter, and appears in Bad Kids of Crestview Academy as Ethel Balducci.

Seven of the eight other actors (Elijah Wood, Sean Bean, Billy Boyd, Ian McKellen, Dominic Monaghan, Viggo Mortensen and Orlando Bloom) all have the same tattoo.

[39] In January 2015, Astin completed the Dopey Challenge at the Walt Disney World Resort which involves running a 5k, 10k, half-marathon and full marathon on four consecutive days.

[49] Astin served as the campaign manager for Democrat Dan Adler, a businessman friend in the entertainment industry, in California's 36th congressional district special election of 2011.

[52] Astin additionally appeared at the 2024 Democratic National Convention during the nominating roll call vote on August 20, 2024, representing Indiana.

[53][54] In September 2024, Astin urged California Governor Gavin Newsom to sign SB 1047, a bill that would require advanced AI models to undergo safety testing before deployment.