The former Shadow Minister was named as Tory MP for Thirsk and Malton in North Yorkshire yesterday after the ballot was delayed for three weeks because of the death of one of the other candidates.
She won comfortably with a majority of more than 11,000, although turnout was low, only half of those entitled to vote casting their ballot.
Liberal Democrat Howard Keal came second, with Labour's Jonathan Roberts third as voters appeared to endorse the two coalition parties.
Speaking after her victory, Ms McIntosh, who was Shadow Minister for environment, food and rural affairs in the last Parliament, said: "It's very humbling to have received the support of so many people.
She said it was a "very exciting time" to be in politics and described the new coalition Government as "a new age of politics".
Ms McIntosh polled 20,167 votes, Mr Keal 8,886 and Mr Roberts 5,168, less than half Labour's projected vote in 2005.
The election was postponed from May 6 after the death in April of the UK Independence Party candidate John Boakes.
The Tory win takes the party's total number of seats in the Commons to 307. The Liberal Democrats, cheered by the second place over Labour, also scored a small net swing against their Tory coalition partners.