Souness' appointment came as a surprise after speculation linking Terry Venables and Steve Bruce with the job.
It is unlikely that Souness was the club's first choice. Shepherd had held talks with Venables about taking charge until the end of the season, while they would have been forced to pay around £3million in compensation to lure Bruce from Birmingham.
Steve McClaren and Sam Allardyce also ruled themselves out of contention.
Souness is not the first manager of Newcastle to have had Blackburn and Liverpool on his CV, though the club's fans will be hoping he will have more success than Kenny Dalglish, who was a hugely disappointing replacement for Kevin Keegan.
The 51-year-old Souness played in in the North-East and is remembered by Middlesbrough fans as one of the club's greatest midfielders.
His no-nonsense approach to playing has been replicated during his managerial career and he has a reputation as an uncompromising character.
At Blackburn, he fell out with star players David Dunn and Dwight Yorke over their hectic social lives.
He also been reprimanded by the Football Association on several occasions for forthright comments about referees.
Temperamental stars, such as England midfielder Kieron Dyer and Welsh striker Craig Bellamy, will find his approach markedly different to Robson's.
Being handed one of the top six jobs in English football represents a remarkable comeback for Souness.
He left Liverpool in 1994 after overseeing the club's worst season in 30 years.
After trying to revive his career overseas with a spell at Galatasaray, he returned to England in 1996 to take charge of Southampton before leaving to work abroad again.
As well as promotion to the Premiership with Blackburn, he also won the League Cup in 2002 although last season was a miserable one which saw Rovers flirting with relegation.
