He has served as director general of the Defence Safety Authority (DSA) since January 2022.

Shell was commissioned in the General Duties Branch of the Royal Air Force on 5 May 1988 as an acting pilot officer.

[11] On 7 January 2022, he was appointed director general of the Defence Safety Authority (DSA) and promoted to air marshal.

[9][12][13] In 2006, Shell was awarded the Bronze Star Medal by the United States "in recognition of gallant and distinguished services during coalition operations in Iraq".

[15] In the 2008 New Year Honours, he was appointed an Officer of the Order of the British Empire (OBE).