Also known as King Rama X, the 10th king of the Chakri dynasty, Vajiralongkorn had been serving as king since the October 2016 death of his father, King Bhumibol Adulyadej, who was on the throne for seven decades.
King Vajiralongkorn, who has reportedly been Queen Suthida's romantic partner for many years, has been married and divorced three times in the past.
He also delivered his first royal command as King, saying, "I shall continue, preserve, and build upon the royal legacy and shall reign with righteousness for the benefit and happiness of the people forever".
Traditionally uttered after a king is crowned, the king's first command serves to capture the essence of his reign.
"The current king has nothing of that and everybody knows about this private life, so he does not approach the popularity of his father".
On Monday, the king will greet the public from the balcony of the Grand Palace and then hold a reception for the diplomatic corps.
Thais wearing yellow shirts - the royal colour - and carrying hats and umbrellas to protect against temperatures reaching 36 degrees Celsius filled the streets outside with many clutching portraits of Vajiralongkorn and shouting "Long live the King!"
King Maha Vajiralongkorn's Royal Land Procession began at 5pm on Sunday at the Grand Palace with Brahmin priests blowing conch shells to signal the moment.
After the coronation, the king will receive members of the royal family, the Privy Council and Cabinet, among other senior officials, who will pay their respects.More news: OnePlus 7 ad drops the phone in water, but you still shouldn't
The teenager is the king's son from his third marriage. Prayuth is seeking to stay on as an elected prime minister after the first elections since the military seized power five years ago.
Full results are due to be announced next week with the ruling junta tipped to return as the civilian government.
Saturday's rituals, which were about transforming him into a "Devaraja", or a divine embodiment of the gods, saw him purified and anointed with consecrated waters from 117 sources.
Although most coronation ceremonies for Thai kings follow Hindu Brahmin traditions, some Buddhist elements were added by the monarch's great-great grandfather King Mongkut, or Rama IV, because he spent 27 years in monkhood before inheriting the throne, scholars said.
The Thai term "Muratha Bhisek" refers to the action of pouring holy water over the head of the King.
The king wore his normal modern royal uniform with a white tunic, a strong contrast with the traditional vestments with gold embroidery that he wore the previous day. During the ceremonies, the king gave alms to saffron-robed, barefoot monks.
The three-times divorced monarch Wednesday his new queen - a former Thai Airways flight attendant and member of his personal bodyguard team - in a surprise wedding that shocked many. Soldiers also fired cannon salutes in the field of Sanam Luang to mark an official beginning of the new reign.
Though present on day one, the king's older sister Princess Ubolratana did not participate.
Absolute rule by kings ended with a 1932 revolution in Thailand that ushered in a constitutional monarchy.