Irwin Harold Mainster

Irwin Harold “Ish” Mainster of Westlake Village, CA passed away peacefully on October 16, 2026, at the age of 80. He was born on October 11, 1946, in Winnipeg, Manitoba, Canada. In May 1965, he moved to Southern California with his parents, Charles and Sylvia, and his two younger brothers, Sheldon and Arthur. While attending UCLA as an undergraduate in the mid-1960s, he was drafted into the US Army and sent to the Vietnam War, returning with a Bronze Star as recognition for his service. Upon his return, he earned an MBA at Cal State Dominguez Hills. Ever an entrepreneur, he used his computer programming skills and business acumen to launch various computer companies, eventually owning and operating his successful medical billing practice, Alexsys Medical Management, for over 35 years.

Irwin was previously married to Lynn (Willenberg) Hamilton, with whom he was blessed with a daughter, Sydney, in 1979 and a son, Scott, in 1982. In February 1994, he married Constance (Morin) Mainster, expanding his role as a father to include her son, Ross Morin (b. 1988). Irwin and Connie remained devoted to each other for 32 years until his passing.

Having lost his mother when he was a teenager, he took caring for family, friends, and business clients seriously and fastidiously. His business was his passion, employing different family members to varying degrees, including his wife Connie, and sons Scott and Ross. Clients became friends, employees became family, and conversations with co-patrons at his regular restaurant haunts turned into recurring rounds on the golf course and annual pilgrimages to watch the Super Bowl in Las Vegas. He loved a hot blackjack table, a cruise to the Caribbean, and watching 4th of July fireworks in Lake Tahoe.

Irwin is preceded in death by his parents, Charlie and Sylvia; his brother, Sheldon; his father-in-law, Armand; his mother-in-law, Sandra; and his godparents, Jennie and Perry Moscovitch.

He is survived by his wife, Connie (Morin) Mainster; his children, Sydney Mainster and son-in-law Justin Field, Scott Mainster and daughter-in-law Marisa (Torres), and son Ross Morin; his granddaughter Parker Mainster-Field and his grandson Cassian Mainster; and his youngest brother, Arthur Mainster, in addition to numerous cousins and extended family.

The family would like to thank Assisted Care Hospice for the care and compassion they showed our loved one in his final days.

Friends and family are invited to attend a celebration of life at Pierce Brothers Valley Oaks on Friday, November 13, 2026, at 10am.