The Sri Lankan off-spinner and the New Zealand part-time bowler were banned in July as part of the governing body's clampdown on players with suspect bowling actions.
"Since being reported earlier in the year, both bowlers have remodelled their bowling actions prior to applying to have their actions re-tested," the ICC said in a statement.
"The results showed that the level of elbow extension measured for all deliveries bowled was now within the 15-degree level of tolerance permitted under ICC regulations."
The duo can still be reported if umpires find them displaying a suspect action and not reproducing the legal technique that was analysed during the re-test.
Senanayake, 29, was included in Sri Lanka's list of 30 probables for next year's World Cup in Australia in New Zealand even before his bowling action was cleared by the ICC.
Williamson, who is primarily a middle-order batsman, also features in New Zealand's preliminary squad for the 50-over tournament, which begins in February.