Graham Westley has bolstered his attacking options by landing the Welsh forward for a five-figure fee.
Cassidy, who is 26 years old, has scored seven goals in 29 appearances during the first half of this season in the National League.
The striker started his career in Wales with spells at Llandudno Junction and Airbus UK before Wolves snapped him up as a youngster in 2010. He went on to make 20 appearances for their first team during his five years on the books at Molinuex.
Cassidy had loan spells away from the Midlands side at Tranmere Rovers, Notts County and Southend United. He left Wolves on a permanent deal in 2015 to join Oldham Athletic.
He spent a season with the Latics before dropping into non-league to join Guiesley, where he impressed enough to earn himself a move to Hartlepool United.
Cassidy scored seven goals in 43 games for the Pools and was then loaned out to Maidstone United before joining Maidenhead last summer.
The well-travelled striker has now moved back into the Football League with Stevenage and will be hoping to score the goals to help them stay in the fourth tier this term.
Boro are currently bottom of the league and a point behind Morecambe in 23rd. Only one team will be relegated from League Two this season which gives Stevenage hope they can survive under Westley.
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