[3][4] Addison initially joined Grey's Anatomy in the Season 1 finale as the estranged wife of Derek Shepherd (Patrick Dempsey).

Although she was originally planned as a recurring guest character, creator Shonda Rhimes grew fond of Walsh's portrayal, which led to Addison becoming a series regular by season 2 episode 7, "Something to Talk About".

The new series saw Addison move to Los Angeles to start fresh, working at the Oceanside Wellness Group, a fictional private practice in Santa Monica, California.

[6] Her mother, Beatrice "Bizzy" Forbes Montgomery, is also a prominent figure, and her brother, Archer, is a renowned neurologist and a successful author.

[7][5] Addison met her future husband, Derek Shepherd, in medical school, and their love story began during their time at Columbia University College of Physicians and Surgeons in New York City.

During her time in New York, she formed close friendships with Sam and Naomi Bennett,[9][10] who would later become her colleagues at the Oceanside Wellness Group in Private Practice.

Although Derek is still angry with her for her affair with Mark, Addison’s arrival stirs up old feelings, and the couple attempts to rekindle their marriage.

[17] Wanting to have a baby, Addison consults her friend Naomi Bennett, a fertility specialist, which introduces the storyline that serves as the backdoor pilot for Private Practice, marking her transition from Grey's Anatomy to the spin-off.

[18][19] Addison eventually decides to leave Seattle and moves to Los Angeles, where she joins the Oceanside Wellness Group, led by Sam and Naomi Bennett.

This officially ends Addison's time as a series regular on Grey's Anatomy, though she makes guest appearances on the show through Season 8.

In September 2021, it was announced via social media that Kate Walsh would reprise her role as Dr. Addison Montgomery in Grey's Anatomy's eighteenth season.

After news of her presence at the hospital spreads online, a large group of protesters gathers outside the clinic due to her work as an OB/GYN.

The situation turns violent when a protester throws a brick with the words "Montgomery murders" written on it through a clinic window.

Addison helps deliver a baby amid the chaos, but while she and her team are being escorted from the clinic, a protester drives a car into her and a pregnant OB/GYN trainee.

In the series, Addison is initially made to feel unwelcome by the other doctors at Oceanside Wellness Group, but she decides to stay and work through the challenges.

[22] In a crossover episode, Addison briefly returns to Seattle Grace to assist with a patient and gives Meredith advice not to let Derek slip away.

[5][27] Archer soon begins working for a rival practice, the Pacific Wellcare Center,[28] and when he suffers multiple seizures and believes he has a brain tumor, Addison asks Derek Shepherd to treat him in another crossover episode with Grey's Anatomy.

In the Season 4 finale, Addison has to operate on her godchild, Maya Bennett, after a car accident, which helps mend her friendship with Naomi.

In the Season 5 finale, Addison and Jake sleep together after Amelia Shepherd delivers a brainless baby whose organs are donated.

When Addison returns home, Sam proposes to her, but Jake is also on his way to her house with flowers and Chinese food, leaving her in a dilemma.

Initially, the character was intended to appear in only a few episodes, but due to the positive reception of Walsh’s performance, Addison quickly became a regular on the show.

Addison's popularity led to the creation of the spin-off series, Private Practice, where Walsh's character moved to Los Angeles to start a new chapter in her life.

[38] Kate Walsh's portrayal of Addison Montgomery on Grey's Anatomy was highly praised by critics and audiences alike.

Joel Keller of AOL TV expressed disappointment with Addison’s evolution during the first season of the spin-off, stating that she "went from strong and funny to whimpering and lovelorn".

Keller appreciated Addison's original strength and wit on Grey's Anatomy and felt her character had become more vulnerable and emotionally dependent in Private Practice.

Despite his criticism, Keller was pleased to see more maturity and depth in Addison’s Season 2 storylines, noting that the quality he admired most in her—"the ability to perform complicated surgical procedures even while everything in her personal life is in chaos"—was reintroduced.

[42] By the time Private Practice concluded, Margaret Lyons of New York Magazine described the character as "fun" and added: "Even when tragedy befell her at every turn on PP, she was still sort of sassy and bright and interesting.

[49][50][51] For her role on Private Practice, Walsh was nominated for Favorite TV Drama Actress at the 37th People's Choice Awards.