On weekdays, 12 trains a day will link Elephant & Castle with Finsbury Park during peak hours only.
Northbound trains will leave Elephant & Castle every 30 minutes between 6.54am and 9.24am in the morning peak, and between 4.24pm and 6.54pm in the evenings.
The journey time between Elephant & Castle and Finsbury Park is 21 minutes.
There will be no weekend service on the new through route, with off-peak trains to and from Sevenoaks continuing to terminate at Blackfriars.
“The new timetable from Sunday 15 May is the starting point for rebuilding our railway, to help our customers get where they want to go while reflecting new travel patterns which have changed significantly, particularly on weekdays," said Angie Doll, chief operating officer of Govia Thameslink Railway.
“Our priority is to support passengers by running reliable services, working closely with our stakeholders to support the economic recovery of both local and regional rail.”
The new Thameslink service has been created by joining up the regular Blackfriars to Sevenoaks service with trains that previously ran between King's Cross and Welwyn Garden City to the north of London.
Thameslink has served Finsbury Park since 2018, but until now only trains that have reached Blackfriars via London Bridge have used the new connection.
This will be the first time passenger-carrying trains approaching Blackfriars via Elephant & Castle have used the new link.
• Since February, passengers from Blackfriars and Elephant & Castle have also been able to make use of a new off-peak Southeastern service to Maidstone.