The Namibia-born, South Africa-raised McGuigan first began working with England during last year's tour of Argentina and the United States, while continuing in his role of defence coach with Prem club Sale.
But from next month, his England position will be made permanent, with the former Scotland centre's responsibilities to include collision skills, the aerial contest and aiding the back three.
"Byron is an excellent addition to our coaching group," Borthwick said.
"Since arriving with us last summer, he has made a strong impact through his detailed understanding of the game and the energy he brings."
He added: "We're excited about the contribution he will continue to make."
England play Nations Championship matches against world champions South Africa, Fiji and Argentina in July.
They are looking to recover from their worst Six Nations, which saw them finish fifth, with England managing just a solitary win in their tournament opener against Wales before losing four games in a row.
But an ensuing Rugby Football Union review of a dismal campaign ended with Borthwick backed to coach England at next year's World Cup in Australia.
On Monday, former England captain Borthwick named his first squad since the review, selecting South Africa-born Benhard Janse van Rensburg for the first time in anticipation of his qualification for England on residency grounds from July 8th.
Janse van Rensburg's inclusion meant there was no room in the 42-man squad for Bath centres Ollie Lawrence and Max Ojomoh.