The big day started after a red-eye Brussels summit, when a German TV journalist surprised Merkel and his own colleagues with a rendition of "Happy Birthday, Liebe Bundeskanzlerin" ("Happy birthday, dear chancellor").
While the reporter's solo performance drew much derision on social media, well-wishers kept showering the leader dubbed "Mutti" (Mum) with birthday greetings ahead of her official party with 1,000 invited guests in the evening.
Mass-circulation daily Bild released a video clip in which celebrities and politicians performed a cover of an old pirate song called "We Love the Storms" for Merkel, and published readers' hand-painted portraits of the woman dubbed Europe's most powerful leader.
While Merkel, also known as Europe's austerity champion, has grown to be loathed by many in crisis-hit southern Europe, Germans resoundingly re-elected her to a third term last year as a vote of confidence in her stewardship.
Merkel's one-time mentor Kohl, Germany's "father of national reunification", urged her to stay the course and remain committed to the European ideal, in an open letter also published in Bild.
"A united Europe and the euro are without alternative for lasting peace and freedom, stability and security on our continent," wrote the 84-year-old.
Kohl, who now rarely appears in public, also pointed to the crucial role of "our American friends", at a time of heightened transatlantic tensions over claims of US spying on Germany.
One of the key architects of Europe's push toward greater unity, Kohl oversaw the 1989 fall of the Berlin Wall and the country's peaceful reunification the following year.
It was at this time, a quarter century ago, that Merkel, a pastor's daughter brought up in communist East Germany, with a doctorate in physics, entered political life.
Under Kohl's guidance she rose through the ranks of the conservative Christian Democrats and became minister for women affairs, then the environment - only to later publicly speak out against Kohl when he was caught up in a slush funds scandal.
Merkel in 2005 became Germany's first female leader and is now in her third term, at the helm of a coalition government with a crushing parliamentary majority.