Smith was 54 when his mother Connie Smith, realising she had twenty-four hours to live, finally admitted to him that he had been adopted. After her death, Smith went in search of his natural mother Peg Kline, whom he finally found. According to Kline's story, Smith was conceived when she was fourteen, the product of rape, and was put up for adoption. Kline eventually married and had two children from this marriage. She never told anyone besides her husband about her first son until she was contacted by Smith via a letter.

She contacted his wife Gail Smith, who mentioned that her son was a prominent Neighbours actor, to which Kline replied, "I've never seen Neighbours in my life". Smith and Kline were reunited and he has built a strong relationship with his mother and two half-brothers, although Smith says that he doesn't feel a mother–son relationship. He calls Kline "one of my best friends". Peg Kline died in May 2005 after a battle with cancer.

Smith first told the story to the public in an exclusive interview on the biographical documentary series Australian Story on March 2005.

Previous to Neighbours, Smith had previously acted in guest roles in drama series such as the Crawford Productions police dramas Homicide, Division 4 and Matlock Police. He had a regular role in long-running Australian television serial Bellbird, made a guest appearance as a police detective in The Box in 1975, and had a semi-regular role in serial Prisoner. Smith was also script editor of Prisoner for most of its eight-year run. He became Associate Producer of Prisoner late in its run.

Neighbours

Smith became best known through his portrayal of Harold Bishop in the soap opera Neighbours from 1987 to 1991, and returning in 1996. He switched to recurring status from 2008 to 2009, becoming one of its longest serving characters.

In 2008, he revealed to the Daily Mail newspaper that his main decision for leaving Neighbours was the on-going abuse that he received from louts near his Melbourne home. He claimed that the abuse was so bad, he had to get the police involved.

In his final Neighbours storyline his character contracted cancer. Harold recovered after treatment and was written out by leaving to travel around Australia.

He was nominated for a Gold Logie Award in 2009. He lost out to Rebecca Gibney.