As for English's legacy? While wary of "pigeonholing" English's 27-year career, McClay cited his management of the country's finances during the Global Financial Crisis and his social investment work as highlights.
He said English would be able to leave Parliament with his head held high.
"It was a privilege to serve as a Cabinet Minister while he was Prime Minister."
With attention now turning to who may succeed English, McClay declined to rule out a leadership bid or discuss any possible candidates.
"All I'll say is that he's only made the decision today," he said.
"This is Bill English's day."
McClay believed the leadership selection process would be "constructive and calm".
National's Rotorua electorate chairman Trevor Newbrook said the announcement took him by surprise.
"I thought he might stand down before the next election, but I wasn't expecting him to stand down at such an early stage."
English's legacy would be the financial position the country was in as a result of his tenure as Minister of Finance, Newbrook said.
The "most obvious" choice of successor was Paula Bennett, but other "strong contenders" included Nikki Kaye, Steven Joyce, Amy Adams, and Simon Bridges, he said.
"We've got a few people that are ranked highly that could do a good job."
Waiariki MP Tamati Coffey said English had "a lot of support in Rotorua for his long service to New Zealand and I wish him all the best".
Rotorua-based New Zealand First list MP Fletcher Tabuteau did not believe anyone would be surprised by the decision.
"I want to acknowledge Bill English for his long-standing contribution to Parliament and New Zealand politics, including his roles as leader of the National Party and Prime Minister.
"I sincerely wish him and his family all the very best for their future plans."
