The Nigeria captain – who had been playing for Chinese side Tianjin TEDA – held talks with Boro manager Tony Pulis last week and underwent a medical at the club yesterday after agreeing personal terms.
Mikel, who has also been interesting Derby County after seeing a move to Crystal Palace fall through last week, has been keen to return to the UK where his wife and children have stayed after his move abroad.
The 31-year-old enjoyed a golden 11-year spell at Stamford Bridge, which saw him win a Champions League title, two Premier League championships, three FA Cups, the Europa League and the League Cup.
The addition of Mikel is likely to be the trigger to some outward movement at the Riverside Stadium with Pulis having spoken about the club’s need to balance the books alongside bringing in new recruits.
The future of Stewart Downing, 34, remains in doubt after it was revealed this week that the former England international is entitled to an improved deal if he makes one more start for Boro.
Downing reportedly earns £35,000-a-week, but will be given a pay rise if he plays one more match due to a clause in the deal agreed when he re-signed for his hometown club in 2015.
The Teessider is likely to be allowed to leave if the club receive an acceptable offer.
Boro have offloaded another of their top earners already this month after allowing unsettled forward Martin Braithwaite – said to be on a weekly wage packet of £50,000 – to join La Liga side Leganes.
Several other players including Rudy Gestede, Ashley Fletcher and Grant Leadbitter have been linked with moves away from Teesside as Boro – who spent just under £50m in the summer of 2017 – seek to ease their wage bill to provide themselves with further room for manoeuvre in the transfer market.
Young Sheffield United defender Callum Semple has completed a permanent transfer to Ross County.
The centre-half has signed an 18-month contract with the Staggies following a successful spell on loan with Queen of the South.