With the money he earned through his acting, they were able to pay off the school fees and eventually started their own family business, a chain of second-hand bookstores around Melbourne.

Johnson's first foray into television started with small roles in various shows including the role of Prince Jobah in The New Adventures of Ocean Girl; as Sally Fletcher's first boyfriend, Gus Bishop, in Home and Away; and in other bit parts including Blue Heelers, Halifax f.p., Stingers and Something in the Air.

His break, however, came in 2001 when he was chosen for the role of the scruffy, womanising writer Evan Wylde in Channel 10’s drama series The Secret Life of Us.

This made Johnson a household name and earned him an AFI Award in 2001 for Best Actor in a Leading Role in a Television Drama Series.

The Secret Life of Us enjoyed consistent success up until the third series, when many major characters left, resulting in a drop in ratings.

It follows the story of the Kirby family; their father Cliff is in the advanced stages of Alzheimer's disease, and relives his disturbing memories of the war and his first love, as a part of his experiences of the present.

Apart from Johnson's appearances on many Australian shows to promote his work, he has also featured on Thank God You're Here and The Panel as well as the ABC documentary The Sum of Sam; documenting his personal struggles and work with Open Family Australia, a youth outreach program co-founded by South Melbourne parish priest Fr Bob Maguire.

In 2019, he competed and won the 16th season of Network 10's Dancing with the Stars, with professional partner Jorja Freeman, and raised $50,000 in prize money for his charity Love Your Sister.

The play follows the ageing veteran after retirement, when he returns to the diaries of his time in a World War II POW camp.

[1] Johnson's other stage credits include: Love Letters, Hotel Sorrento, The Present, The Snake Pit, Mad Woman’s Fountain, Life During War Time and The Pink Panther Strikes Again.

[12] Johnson and Woodlands' mother Kim then endured a bitter legal tug-of-war for two months with Lainie's estranged father to bury her close to her chosen home and those she loved.

[15] The court was told that at the time of the offence Johnson was receiving treatment for depression following the suicide of his long-term partner Lainie Woodlands.

[citation needed] In May 2008, Johnson gave his first interview since the death of Woodlands to ABC Television's Australian Story – The Sum of Sam.

He talked about the turmoil of the last three years – and his life changing involvement with Open Family Australia, a charity that works with vulnerable young people.

In 2013, Johnson began riding 15,000 km on a unicycle in a year-long attempt to break the Guinness World Record and raise $1 million for the Garvan Institute of Medical Research to find a cure for breast cancer.

[21] His stated mission is to remind every Australian woman about the need to be 'breast aware', in an effort to promote early detection and improve survival rates, via his charitable foundation, Love Your Sister.

[23] In 2016, Johnson was awarded the Medal of the Order of Australia (OAM) for service to cancer research support organisations, and to the performing arts.