Stewart, who also runs his own firm, Bob Stewart Associates, will extend Arnold's offering into the issues management arena, targeting potential and existing clients.
While he said that a full merger had not yet been discussed, he hinted at the possibility of further co-operation in the future.
The relationship will also see Arnold Group benefiting from the presence of Bob Stewart Associates in the US.
Arnold Group director Tim Arnold said: 'We are asked to do a lot of crisis and issues management work and it's great to have Bob on board to add expertise in this area.'
Stewart formerly worked at Hill & Knowlton as a senior consultant in the public affairs division after leaving the army in 1995. He left three years later to run satellite radio company WorldSpace UK as MD.
'Where either of us has a gap in our offering, we can bring in the other's expertise,' said Stewart.