Beginning a career as a professional child model and actress at the age of eleven, Haslam is best known for her role as "Becky Thatcher" on the Hanna-Barbera children's television series, The New Adventures of Huckleberry Finn which originally aired on NBC from 1968 to 1969.

[2] In the subsequent years, Haslam worked as a child actress and model, appearing in print advertorials and television commercials for Diet-Rite Cola, Mattel Toys, and Downy Fabric Softener among others, before landing the role that would make her a star.

[2][4] After several years as a working actress in commercials, Haslam landed the role of Becky Thatcher on the Hanna-Barbera television series The New Adventures of Huckleberry Finn, loosely based on the characters from the Mark Twain novel.

[3][5][6][7] 14 years old when she landed the role, Haslam co-starred alongside Michael Shea as Huck Finn and Kevin Schultz as Tom Sawyer, navigating weekly adventures within an animated world as they attempted to outrun a vengeful "Injun Joe", played by Ted Cassidy.

[5] In a departure from the network's usual Saturday morning cartoon schedule, Huckleberry Finn aired Sunday nights and made its three young stars popular teen idols of the era.