Who was the longest serving Pope from the start of the 20th century?

Pope John Paul II (18 May 1920 – 2 April 2005) was head of the Catholic Church and sovereign of the Vatican City State from 16 October, 1978 to 2 April, 2005 (26 years, 168 days). He was elected pope by the second papal conclave of 1978, which was called after Pope John Paul I, who had been elected in August to succeed Pope Paul VI, died after 33 days. Cardinal Wojtyła was elected on the third day of the conclave and adopted the name of his predecessor in tribute to him. John Paul II is recognised as helping to end Communist rule in his native Poland and eventually all of Europe.

John Paul II was the second longest-serving pope in modern history after Pope Pius IX, who was Pope from June 16, 1846 until 7 February, 1878 (31 years, 236 days).

Born in Poland, John Paul II was the first non-Italian pope since the 16th-century Pope Adrian VI. John Paul II's cause for canonisation commenced one month after his death with the traditional five-year waiting period waived.