Of Meneldil's life before the War of the Last Alliance we know almost nothing, but it is possible to deduce some of the most important incidents in that time. He was born in the very last years of Númenor, and was at most twenty years old at the time of the Downfall. He would have seen the drowning of the island with his own eyes, from the deck of one of his father's ships, and then was propelled by the rushing water to the shores of Middle-earth.
What part he took in the building of the Realms in Exile is a mystery. During this time, he would have been considered a relatively minor nobleman. As the fourth child of Elendil's younger son, he was at best seventh in line to the succession, and the prospect of his becoming King was remote. After more than a century, Sauron rose again in the east, triggering the great events of the War of the Last Alliance. It is to be presumed that Meneldil played his part in that War - a war in which he lost his father Anárion - but we have no specific details of his deeds.
Before departing for the North, Isildur spent a year ordering matters in Gondor, and instructing Meneldil in his new duties. During this year, Isildur and Meneldil journeyed around the borderlands of the South-kingdom, defining its bounds. It was while returning from their travels that they came upon Amon Anwar, the Halifirien,which at that time stood almost at the central point of the country. Isildur chose that hill as the resting place of his father's remains, and the Tomb of Elendil that he placed there was to last long after Meneldil's line had failed.
It was the beginning of Yavannië4 when Isildur set out from Osgiliath, leaving the South-kingdom in Meneldil's hands. We are told that Meneldil was not disappointed to see the High King and his sons leave for the North, and that he hoped to rule Gondor himself for a long time. Far-sighted as he was, though, even Meneldil couldn't have foreseen the events that were to unfold. A month after setting out, Isildur was set upon by Orcs and killed, together with his three elder sons. The Kingship in the North fell on Isildur's only surviving son, eleven-year-old Valandil. This unexpected turn of events transformed Meneldil from a mere caretaker into the true King of Gondor.
After the upheavals of III 2, Meneldil was to rule Gondor for many years,but during this period he effectively disappears from history. With Sauron utterly defeated, his reign was a peaceful and quiet one. The only historical events recorded during this time are the wedding of Elrond to Celebrían, and the birth of their twin sons Elladan and Elrohir. These would have been important events in the North, but we cannot be sure if the Gondorians were even aware of them.
Meneldil ruled in Gondor for one hundred and fifty-six years, and was succeeded by his son Cemendur.
The unlikely ascension of Meneldil to Gondor's throne had a vast impact on history. It resulted in the kingdoms of Arnor and Gondor becoming politically separate entities, with Arnor ruled by the descendants of Isildur, while Gondor'sKingship was inherited by the descendants of Meneldil. It is far from certain that this was Isildur's intention, or even that Meneldil had any legal claim to the throne at all.
When Valandil became King of Arnor, he should legally have inherited the Kingship of Gondor too, and we might have expected Meneldil to acknowledge the authority of the new High King. Meneldil did no such thing. He seems to have taken advantage of Valandil's youth and inexperience to establish himself as a King in his own right, effectively 'declaring independence', and removing Gondor from Arnor's control. This could even be seen as Meneldil's usurping the throne of Gondor, and it is surprising that there is no record of any dissent at the time.
We know that Meneldil was a shrewd and intelligent man, and it seems more than plausible that he simply invented this story of Isildur's 'relinquishing' Gondor to his rule. It certainly isn't true, since we have evidence that Meneldil fully expected Isildur to return to the South (in The Disaster of the Gladden Fields, we are told of Meneldil's hope that '...affairs in the North would keep them [Isildur and his sons] long occupied.') He would hardly have thought in these terms if he truly imagined that his uncle had given him the Kingship of Gondor in perpetuity.
The date of Meneldil's birth appears only in The History of Middle-earth volume XII, The Peoples of Middle-earth. It cannot therefore be considered completely reliable, especially since an alternative birth date of II 3318 also appears in that source. According to that alternative version of events, Meneldil was the last Man to be born in Númenor before it was destroyed the following year, but this story seems to have been abandoned. Later drafts apparently supersede this, instead naming Isildur's eldest son (there called 'Kiryandil', but later renamed Elendur) as the last Númenórean, and making Meneldil's birth nineteen years earlier: II 3299.
In an added note that appears in The Peoples of Middle-earth, Meneldil is identified as Anárion's '4th child', but we have no information of any kind regarding his three elder siblings. Most likely, all three were daughters, or brothers who perished in the War of the Last Alliance (as indeed had Anárion himself). We do know that Meneldil was a rather ambitious Man, and was eager to take on the rule of the South-kingdom. If Meneldil was originally fourth in line to the throne, then the disappearance of his elder siblings, combined with his known ambitious character, might even lead us to suspect dark deeds in Gondor's earliest history!
It isn't entirely clear why Isildur didn't follow his father's example, and commit the rule of the South-kingdom to his own sons. All four had survived the War of the Last Alliance, and three (Elendur, Aratan and Ciryon) were on hand to take over the task immediately. Perhaps the fact that Meneldil was elder than any of these (he was probably 143 at the time) swayed Isildur's choice.