From 1809 to 1812, Lenné took many study trips, paid for by his father, which took him to Southern Germany, to France, and to Switzerland. In 1811, he completed a long internship in Paris with Gabriel Thouin, who was then the most famous garden architect in Europe. This made him a master landscaper. On another of these trips, Lenné made the acquaintance of the creator of the English Gardens in Munich, the landscape gardener Friedrich Ludwig von Sckell, who would have a lasting influence of Lenné's work.
In 1812, Lenné followed his father to Koblenz, where he had been named Director of the Gardens by the Prefect Jules Doazan. Later in that year, Lenné became active at Schloss Schönbrunn, where he would remain until 1814. He then returned to Koblenz, where he was give private garden commissions until 1815. The "Erweiterung der Festungsanlagen veranlasste ihn zu einem Plan für die Verschönerung der Stadt mit Gartenanlagen", which was not carried out because of a lack of funds. In 1816, he returned to Potsdam after the suggestions of Prussian forestry official Georg Ludwig Hartig and General Graf von Hacke. There he received the position of Assistant Gardener to the Court Garden Director in Sanssouci.
Still working as an assistant gardener, Lenné received a commission in the Spring of 1816 from the Prussian Chancellor Karl August von Hardenberg to renovate the grounds around his country house at Klein-Glienicke. This work on Glienicke Palace, which would later become Prince Carl of Prussia's residence, laid the groundwork for Lenné's designs for the surrounding area of Potsdam, which he wanted to turn into a Gesamtkunstwerk. The upgrades of the Glienicke grounds followed - in close cooperation with the architects Karl Friedrich Schinkel, Ludwig Persius, and Ferdinand von Arnim - those of others such as the grounds of Böttcherberg and its opposite, Babelsberg Park, which was completed by Prince Hermann von Pückler-Muskau. Characteristic of Lenné's work are versatile Sichtachsen - a horicultural stylistical device - which he applied at Sanssouci Park and elsewhere. As part of the Berlin-Potsdam cultural landscape, which stretches from the Pfaueninsel to Werder, many sites of Lenné's work are World Heritage Sites, which has been under the protection of UNESCO collectively since 1990.
In 1823, the Gardener Academy in Schöneberg and Potsdam was founded under his management. Here for the first time schooling in garden architecture was taught in a scientific manner. In 1828, Lenné was named the sole Garden Director and in 1845, Prussian Garden Director-General. The Akademie der Künste made Lenné an honorary member.
In 1840, the recently enthroned King Friedrich Wilhelm IV assigned the urban planning of Berlin to Lenné. One of his most important achievements in this role survives in the building of the Luisenstädtischer Kanal, constructed in 1852, between the Landwehrkanal and the River Spree in Berlin-Kreuzberg. The canal's design was based on plans by Chief Building Officer Johann Carl Ludwig Schmid.
Despite centering his life around Potsdam and Berlin, Lenné remained attached to his Rhenish homeland and contributed to the further beautification of Koblenz, particularly in the Rheinanlagen, which was under his management until 1861. His love of his work on the Rhine and Mosel made him decide to build the residence named for him, the Lenné-Haus, in which he wished to spend the evening of his life; however, the manner of his death did not allow this. Lenné's last resting place is at the Bornstedter Cemetery in Potsdam.
Busts of Peter Joseph Lenné are located at the Bonn Botanical Garden, at the bank of the Rhine (Alter Zoll), in the Landschaftspark Petzow that he himself designed, in Feldafing Park, in the Kaiserin-Augusta-Anlagen in Koblenz (copy of a bust by Rauch). A new bust was finished by Bad Homburger sculptor Otto Weber-Hartl.
Today Lenné's descendants still work as architects.