Tewes' first break came in mid-1974 when she starred in a Lipton Ice Tea commercial, allowing her to join the Screen Actors Guild and register with an agent with the prospect to work on film projects.

Tewes was soon cast in roles on the prime time TV series Charlie's Angels ("Angels in Chains"), Vega$ (My Darling Daughter), and Family ("Mirror, Mirror on the Wall...") as Jill Redfield, a disenchanted Pasadena debutante. However, it was her role in Starsky & Hutch ("Starsky and Hutch Are Guilty") as Sharon Freemont, an assistant district attorney, which brought her to the attention of Aaron Spelling.

Tewes was cast for the role of cruise director Julie McCoy on The Love Boat, selected from more than 100 actresses who auditioned. She starred in the third and final pilot of the show, cast the day before production began on the RMS Queen Mary in Long Beach. Tewes recalls the pilot episode:

"I had to borrow money to get a new tire, because my ’62 Volkswagen Bug was not going to get to San Pedro…. That first day, standing there in the little outfit, and I had to say, ”Hi, welcome aboard, I’m Julie McCoy, your cruise director” a gazillion times. But I kept screwing it up and saying, ”Hi, welcome aboard, I’m Julie MacLeod…” because I was talking to Gavin MacLeod and I was so excited."