Simmons was born in London, England and grew up in Bristol. He was educated at Wellsway School, a state comprehensive school in the town of Keynsham in Somerset in South West England, followed by Goldsmiths College, a constituent college of the University of London, where he obtained a Bachelor's degree. He has one sister, actress Mika Simmons.
Simmons joined ITN as a reporter for IRN in 1996, before becoming a sometime general reporter on its ITV News service in 1998. While remaining with the organization he intermittently worked for Sky News, 5 News and ITV London's London Tonight. On 30 September 2010, it was announced that he would be promoted from crime correspondent to UK editor.
In 2003, he was nominated for a Royal Television Society Award.
On 16, 23 and 30 September; 7 October 2011, Simmons guest presented ITV Breakfast programme Daybreak.
On 27 August 2012, he left ITN to join NBC News as a London-based correspondent.
In 2013 Simmons covered the resignation of Pope Benedict and selection of Pope Francis's for NBC's TODAY from Rome.
In 2014 Simmons covered the Sochi Winter Olympics for NBC <
In November 2015 he covered the Paris terror attacks.
In March 2016 he covered the Brussels terror attacks for NBC News, appearing on Today, Nightly News and MSNBC. In August 2016 he covered the Rio Summer Olympics for NBC's Today show.
In July 2016 he was nominated for an Emmy award in the category 'Outstanding Coverage of a Breaking News Story in a News Magazine' for his contribution to a Dateline NBC special 'Terror in Paris'.
In July 2017 he was nominated for an Emmy award in the category 'Outstanding Coverage of a Breaking News Story in a News Magazine' for his contribution to a TODAY Show special 'Terror in Brussels'.
Simmons additionally works as a facilitator of live debates and events.
On June 28, 2017 and July 31, 2017, Simmons co-hosted Today with Kathie Lee and Hoda substituting for Kathie Lee Gifford.