The 26 year-old was due to fight for the full WBO title against Ricky Burns late last year until the accident ruled him out and Walsh has now made his claim once again with an impressive ten round win.
Walsh was content to stand and trade with the ageing Harrison, 35, who tired towards the end after a huge effort in the first six rounds and may now have nowhere else to go with his stalled career.
The Scotsman is nowhere near the old full capacity he was when reigning as WBO featherweight king and his stint in a Spanish prison has obviously taken its toll, along with his public party lifestyle.
After taking over from the seventh round onwards, the judges handed Walsh a 98-92, 97-32 (twice) victory as the southpaw took his record to 14-0 and moved closer to a big domestic battle with Burns.
Also on the bill at Wembley Arena, Paul Butler picked up the Commonwealth super-flyweight title with a comprehensive fiftth round technical knockout victory over Yakub Kareem, whilst Mitchell Smith stopped Gavin Reid in the first round.