Ling was formerly associate director in the firm's public affairs team in London, where he worked for a number of ministers in the previous coalition Government as well as advising the Liberal Democrats on economic and disability policy. Before joining Grayling in 2012 he worked for disability charity Scope as head of campaigns.
Longdin has worked for Grayling for seven years, and was previously an associate director in the Midlands team. The former journalist's expertise lies with consumer and automotive PR, the firm says.
The pair will be based in Birmingham, and will sit on the company's UK management board, and will work with Sally Rhodes, the firm's MD of UK regions, nations and Ireland, to enhance the Midlands offer to existing and new clients in a range of sectors.
Rhodes said: "The recent economic growth to the Midlands means there is incredible potential for big things to happen in this region and Grayling is well positioned to support clients who are at the crux of growth in the area."
Earlier this month the firm also made two senior digital and creative appointments in London.