The short answer is Finland had no official involvement in the siege of Leningrad. They did have a significant indirect involvement, however. Also, the Finns did do what little they could in an unofficial manor in regards to helping individuals who came to their border looking for food. What they could do didn’t amount to much but they were in a no-win situation in this instance.
Finland declared war on Russia separately from Germany in June, 1941, just days after the German invasion of Soviet Russia. The Finns were aware of the planned German attack and in return for help in buying arms and in fighting the Soviets, they did allow German troops in Finland just before the attack and on newly conquered Soviet territory at the Finnish border after.
Here is my shortened version of the whole story as posted on:
Finland was in 1939 and through the war years a small democratic country that along with Sweden had the misfortune of being located next to two of the most powerful totalitarian regimes in the world (Soviet Union and Germany).
In November, 1939 the Soviet Union invaded Finland because the Finns refused to cede Finnish territory—and meet other demands—made by the Soviets. The war was deemed illegal by the League of Nations and the Soviet Union was expelled as a result.
Though the Soviets had an army three times as large, a hundred times more tanks and thirty times more aircraft, the Finns fought well. Finnish forces were fighting for their own country and the Soviet army command structure had been decimated by the numerous purges by Stalin in the years leading up to the war. As a result, Finland repelled Soviet advances for several months. Eventually, the sheer weight of men and materials allowed the Russians to conquer more than the territory they had originally demanded.
Due to the unexpected costs of men and equipment to win the relatively small gains, Moscow accepted a peace initiative offered by Finland. It is widely agreed that the Soviets original intentions were to conquer all of Finland but bit off more than they could easily chew at the time.
On March 12, 1940 a peace treaty was signed. As a result, the Finns ceded 11% of their overall land mass, approximately 30% of their economy and had to relocate 442,000 people who had lost their homes and farms.
During the subsequent run-up to the German invasion, Finland was once again being pressured by the Soviets to allow them to operate Finnish nickel mines in the Petsamo region for Soviet use. The Finns refused. At the same time they demanded the Finns allow them access to the Finnish railways to transport Soviet troops to and from their new military base on former Finnish territory. Again, the Finns refused.
Seeing what was coming, Finland began seeking assistance in building military and related equipment. They sought support from democracies in the League of Nations and other Nordic countries. The support that had been offered by Great Brittan was not available after the Nazi invasion of Norway and Denmark in 1940. After that, Britain cut all trade and traffic with Scandinavian countries. Guess who was the only country left to buy military supplies from? That’s right, it was Germany.
In addition, the Germans supplied Finland with information as to what the Soviets intentions were regarding Finland. That being, the Soviets told the Germans they intended to handle Finland the same way they handled the other Baltic States, by installing puppet regimes that were directed by Moscow. Lastly, the Soviet Union had made it clear to the Finns that in case of war with Germany, they intended to attack Finland immediately. This is exactly what they did when the time came as well.
Finland was a democracy and preferred help from the democratic nations that would soon be allied against Germany. However, that was impossible in 1940 and 1941. Therefore, in order to ensure their own country’s’ continued existence, they worked with the German government on certain military common interests. They never had any official agreement, nor provided other official assistance to the Nazis that axis allies did. No, they did not allow Germany any say in Jewish affairs in Finland—and yes, the Germans did ask.
In short, the Fins made it clear to Hitler they were at war with Russia for their own reasons. Though it was mutually beneficial for the Finns and the Germans, they were not allied in the way other nations were.