The nine employees were made redundant when Ilex closed earlier this year.
Ilex was set up in 2003 to regenerate two former Army bases but was shut down in March amid accusations it failed to deliver enough investment and jobs.
The ex-workers taking the legal action have accused the Northern Ireland Executive of breaching employment law.
A spokesperson for the executive said it could not comment as the case was before an industrial tribunal, which is due to be heard early next year.
Ilex was criticised in an independent review in July 2015 over failings in the redevelopment of Fort George Army base and Ebrington Barracks in Derry.
The report said the lack of progress in infrastructure investment, specifically in developing business accommodation, was particularly concerning.
Among its successes, however, the company pointed to its role in helping to build Derry's Peace Bridge and the opening of the Northern Ireland Science Park on the Fort George site.
In March, 18 Ilex workers were made redundant and the two bases were handed over to Derry City and Strabane District Council.
Nine of those workers are taking action against the Northern Ireland Executive claiming their jobs should also have been transferred to the council.
They have accused the Northern Ireland Executive of breaching a law which ensures workers move from one employer to another when a business changes hands.
Former staff at a government-run regeneration body in Londonderry are taking a case against Stormont over the way they lost their jobs.