Watch: Mick Lynch speaks as UK train strikes continue
Around 40,000 RMT members walked out as part of the strike — the latest of several taking place this summer — with just 20% of rail services operating across the UK.
Delaney posed alongside Lynch for a picture shared on social media, in which he referred to the duo as "two nice men wondering if you'd care to join them at the picket".
The 45-year-old star, who has lived in London for several years, appeared in front of a banner featuring a quote by famed trade unionist James Larkin, reading: "The great appear great because we are on our knees. Let us rise."
— rob delaney (@robdelaney) July 27, 2022
Delaney has been a vocal supporter of left-wing politics in both the UK and his native USA, endorsing Bernie Sanders in recent presidential races and Jeremy Corbyn during his time as Labour Party leader.
The current strikes by the RMT arose from the ongoing dispute over pay and conditions that led to several days of strikes in June.
Speaking on the Today programme, Lynch spoke about the lack of progress made in discussions with Network Rail and other train companies.
He said: "It's wishful thinking or just plain propaganda if they think the offer they've put to us would be any way acceptable to my union or its members."
Delaney initially rose to fame at the tail end of the noughties as one of the first comedians to make heavy use of Twitter in sharing his jokes.
He co-created the 2015 sitcom Catastrophe with Sharon Horgan, in which they play an American man and Irish woman who have a child together after a fling, resulting in him moving to the UK to join her.
The show ran for four series and came to an end in 2019, having won a BAFTA for its writing and received an Emmy nomination.
On the big screen, Delaney is best known for his performance as X-Force member Peter in 2018 superhero sequel Deadpool 2.
Watch: Rob Delaney on being slapstick punching bag in Home Sweet Home Alone