It was a photo opportunity for the history books: an Islamist politician from Israel’s Arab minority grinning alongside a far-right Jewish leader and his allies, moments after endorsing him as prime minister and handing him a governing majority.

Common cause against Prime Minister Benjamin Netanyahu helped bring Mansour Abbas into the political fold late on Wednesday, his tiny Islamist faction securing a paper-thin majority for Jewish parties hoping to unseat Israel’s longest serving premier.

The United Arab List would be the first party drawn from the country’s 21% Arab minority – Palestinian by heritage, Israeli by citizenship – to join an Israeli government, whose parliamentary approval is still pending...