Alyssa Jayne Milano was born on December 19, 1972, in the Bensonhurst neighborhood of Brooklyn, New York City, to fashion designer and talent manager Lin Milano and film music editor Thomas M. Milano.

Of Italian descent and raised Catholic, she relocated with her family to Great Kills on Staten Island after a neighborhood shooting, and later to Los Angeles for her career, where she has a younger brother named Cory.

Alyssa Milano began acting at age seven when her babysitter took her to an audition for the national touring company of the musical Annie, selecting her from over 1,500 girls for an 18-month tour that launched her career in television commercials and off-Broadway productions like the first American musical adaptation of Jane Eyre.

At age 10, Alyssa Milano landed her breakthrough role as Samantha Micelli, the daughter of Tony Danza's character, on the sitcom Who's the Boss?, which ran from 1984 to 1992 on ABC and established her as a teen idol while her family moved to Hollywood. 

She made her film debut in the 1984 coming-of-age drama Old Enough, which won First Prize at the Sundance Film Festival, and followed with roles like Jenny Matrix in Commando opposite Arnold Schwarzenegger in 1985.

In the 1990s, she pursued edgier roles including a teenage prostitute in Where the Day Takes You (1992), Amy Fisher in the TV film Casualties of Love: The Long Island Lolita Story (1993), and nude appearances in films like Embrace of the Vampire (1995) to shed her "nice girl" image, while also releasing four bubblegum pop albums that went platinum in Japan.

Alyssa Milano gained renewed prominence as Jennifer Mancini on Melrose Place from 1997 to 1998 and especially as Phoebe Halliwell, one of the three witch sisters, on Charmed from 1998 to 2006, where she also served as a producer starting in season four.

Later television roles included Billie Cunningham on My Name Is Earl (2007–2008), Savannah "Savi" Davis on Mistresses (2013–2014), and Coralee Armstrong on Insatiable (2018–2019), alongside films like Hall Pass (2011) and voice work in Ghostbusters: The Video Game (2009).

She expanded into producing with projects like Sundays at Tiffany's (2010), created the comic book series Hacktivist (2013), hosted the podcast Alyssa Milano: Sorry Not Sorry (2019), and is set to play Roxie Hart in Chicago on Broadway in fall 2024.

A prominent activist since the late 1980s, when she befriended AIDS patient Ryan White and appeared with him on The Phil Donahue Show to combat stigma, she has served as a UNICEF Goodwill Ambassador since 2009, conducting field work in Angola, India, and Kosovo.

She relaunched the #MeToo movement on Twitter in October 2017, encouraging survivors of sexual harassment to share their stories, and has advocated for voting rights leading to her 2021 arrest outside the White House, reproductive rights, gun control via #NoRA, and humanitarian causes including PETA, neglected tropical diseases, and Ukraine's UNITED24 initiative in 2024.

Alyssa Milano launched the Touch by Alyssa Milano apparel line for female sports fans in 2007, wrote Safe at Home: Confessions of a Baseball Fanatic (2009) and Sorry Not Sorry (2021), and endorsed politicians like Bernie Sanders, Hillary Clinton, and Joe Biden.

In her personal life, she dated actor Corey Haim from 1987 to 1990 and was engaged to Scott Wolf in 1993 before their split, during which she had two abortions. Her brief 1999 marriage to singer Cinjun Tate ended in divorce that year, and she married CAA agent David Bugliari in 2009, with whom she has a son born in 2011 and a daughter in 2014.

Residing in Bell Canyon, California, with various pets, she has managed dyslexia since childhood, disclosed a long COVID diagnosis in 2022, and was involved in a 2021 car crash as a passenger.

Alyssa Milano successfully sued pornographic websites in 1998 for unauthorized nude photos and settled a 2017 lawsuit against her business manager.​