The agency pitched against Interpublic's Deutsch LA, Omnicom's TBWA\Chiat\Day and Carmichael Lynch, also part of Interpublic, to win the account, despite not being on the original list of 20 agencies that RadioShack had requested information from.
The account was formerly handled by RadioShack's in-house agency Circle R Group, but went up for review in February after the appointment of a new chief marketing officer Don Carroll.
Carroll said that Arnold had been considered for the pitch after sending in unsolicited information to the marketing team.
"The simple two-page letter absolutely hit the mark by outlining what they perceived to be RadioShack's brand truths -- the good and the bad. The letter was from Arnold," he said.
He added that the insight was uncanny, and clearly showed the team that this agency understood the brand without any previous contact.
"The letter intrigued us so much that we had to send an request-for-information," Carroll said.
RadioShack has nearly 7,000 stores and is based in Fort Worth, Texas. The company's media account, held by Carat's Aegis, is not affected by the review.
Ed Eskandarian, chief executive officer of Arnold Worldwide, said: "The company has an exciting vision for the future, and we believe our integrated communications and top creative will capitalise and reinvigorate its brand with its customers."
If you have an opinion on this or any other issue raised on Brand Republic, join the debate in the .